Patent GB-680709-A (GB)
Title: Improvements in or relating to the production of moisture-proof sheet wrapping materials
Inventor: BERRY WILLIAM
IPC: B29C48/08
Filed: 1949-03-08
Abstract: <PICT:0680709/IV (a)/1> <PICT:0680709/IV (a)/2> Films are produced by depositing electrically charged colloidal particles of polymers or copolymers of vinylidene chloride from aqueous emulsions in layers on to base sheets of good electrically conducting material having an electric charge of opposite sign to that of the particles, and stripping the deposited layers from the base sheets.  The deposited layer after stripping may be dried and consolidated by heat, or if desired, dried and consolidated prior to stripping.  The emulsion may contain 30 per cent solids or more, have a pH of 1-5 (preferably 2) and have a temperature between 20 DEG  and 40 DEG C., preferably 25 DEG  C.  The current used may be such that the deposition is completed in about 10 seconds.  The base sheet is preferably a smooth band or plate of nickel, chromium plate or stainless steel, unsuitable materials being copper, iron, lead and aluminium.  A low voltage should be employed with a potential gradient of 25 volts per inch, although gradients of from 5 to 500 volts per inch may be used. With negatively charged particles, a positive charge is given to the base sheet (anode), a negative charge being applied to another metallic plate immersed in the emulsion (cathode).  To prevent coating taking place on both sides of the anode, the anode face remote from the cathode may be coated with a non-conducting water-impervious coating, e.g. silicone grease.  Preferred copolymers contain 80-98 per cent of monomeric vinylidene chloride.  The remaining ingredient or ingredients may be vinyl chloride, vinyl acetate, styrene, acrylonitrile or an alkyl acrylate or alkacrylate the alkyl group(s) of which contain not more than four carbon atoms, such as butyl acrylate or butacrylate.  The base sheet may be coated with a thin layer of a grease such as micro-crystalline wax to facilitate stripping.  The deposited layer may be dried and consolidated at 120 DEG C. for 4 minutes prior to stripping, then stripped and extended on a frame and the drying completed at 140 DEG C. for 2 minutes.  The dried film may be stretched if desired.  In an example an emulsion is prepared by refluxing at 39 DEG C. for two hours a mixture comprising 9 parts of vinylidene chloride, 1 part of acrylonitrile, 0.06 part ammonium persulphate, 0.03 part sodium metabisulphite, 1.0 part of a 20 per cent aqueous solution of the sodium salt of C12-C18 alkyl sulphates and 12.5 parts of water (all parts by weight).  Two plates of "Monel" metal (Registered Trade Mark), held at 1/4 inch apart, are immersed in the cooled emulsion and are connected to a 6-volt accumulator for 15 seconds.  The anode is coated on both sides. The coating on the side facing the cathode is stripped off, and dried and consolidated by radiant heat.  In example 5 the anode is formed as a stainless steel drum.  In an apparatus suitable for continuous operation, a nickel plated drum B, supported on and insulated from the framework C of the machine, forms the cathode and is negatively charged, the framework being earthed. Two flat rubber tyres D are placed at the edges and an endless nickel band E is pressed into contact with the tyres to form a cell F.  The band is positively charged by its connection with the machine. The emulsion flows into the cell F.  After three-quarter of a revolution, the band leaves the drum and returns over rollers G. Water is evaporated from the deposited film by heater H, the film fused by heater J, and finally cooled by an air blast K.  The film is then stripped, and wound on to roller L.  The drum B and two of the rollers G are positively driven.  Impoverished emulsion is removed from the cell continuously on the surface of the deposited film leaving the cell, and fresh emulsion is fed from the tank N. One roller G is adjustable to take up any slack in the band E. When stopping the process, the electric current and supply of emulsion are cut off, the adjustable roller moved to slacken the band E, thus emptying the cell F into the drip tray M. Specifications 654,342 and 655,732 are referred to.